Raman Study of Trigonal-Cubic Phase Transitions in Rare-Earth Aluminates

The Raman spectra of LaAlO₃, PrAlO₃, and NdAlO₃ have been observed at temperatures from 10 to 1135^∘{}K. Each of these materials evidences a nominally second-order trigonal cubic phase transition; that in LaAlO₃ has previously been analyzed by other workers on the basis of x-ray, EPR, and inelastic neutron scattering; however, evidence for the transitions in PrAlO₃ [at (1320±{}20)^∘{}K] and NdAlO₃ [at (1640±{}30) ^∘{}K] is presented here for the first time. In each material, the phase transition is due to the collapse of the Γ₂₅ mode at the R point of the high-temperature cubic Brillouin zone. Modes associated with Γ₂₅ and Γ₁₅ are observed in the trigonal phase in each crystal.